Diagnostic markers are important for early diagnosis of many diseases, as well as predicting response to treatment, monitoring treatment and determining prognosis of such diseases.
Diagnostic assays are frequently performed on samples removed from patients. Preferably, these samples are obtained in a minimally invasive manner, for example serum or urine samples. However, such assays can only provide information concerning the state of the marker in the particular sample. They are not able to provide direct information concerning the exact location of metastases and/or the degree of tumor shrinkage, for example.
In vivo imaging technologies provide non-invasive methods for determining the state of a particular disease in the human body. For example, entire portions of the body, or even the entire body, may be viewed as a three dimensional image, thereby providing valuable information concerning morphology and structures in the body. Such technologies may be combined with the detection of particular diagnostic markers, particularly molecular biomarkers, in order to provide information concerning the state of a disease or pathological condition in the human body.
Such imaging is also expanding because of advances in technology. These advances include the development of new contrast agents or labels, such as radiolabels and/or fluorescent labels, which can provide strong signals within the body; and the development of powerful new imaging technology, which can detect and analyze these signals outside the body, with sufficient sensitivity and accuracy to provide useful information. The contrast agent can be visualized in an appropriate imaging system, thereby providing an image of the portion or portions of the body in which the contrast agent is located. The contrast agent may be bound to or associated with an antibody, for example, and/or with a peptide or protein, or an oligonucleotide (for example for detection of gene expression) or complex of these with one or more macromolecules and/or other particulate forms.
The contrast agent may also feature a radioactive atom which is useful in imaging. Suitable radioactive atoms include technetium-99m or iodine-123 for scintigraphic studies. Other readily detectable moieties include, for example, spin labels for magnetic resonance imaging (MRI) such as iodine-123 again, iodine-131, indium-111, fluorine-19, carbon-13, nitrogen-15, oxygen-17, gadolinium, manganese or iron. Such labels are well known in the art and could easily be selected by one of ordinary skill in the art.
Standard imaging techniques include but are not limited to magnetic resonance imaging, computed tomography scanning, PET, SPECT and the like. For diagnostic in vivo imaging, the type of detection instrument available is a major factor in selecting a given contrast agent, such as a given radionuclide and the gene product it targets (mRNA, protein and the like). The radionuclide chosen must have a type of decay that is detectable by a given type of instrument. Another important factor in selecting a radionuclide for in vivo diagnosis is that its half-life be long enough that it is still detectable at the time of maximum uptake by the target tissue, but short enough that deleterious radiation of the host is minimized.
Exemplary imaging techniques include but are not limited to PET or SPECT, which are imaging techniques in which a radionuclide is synthetically or locally administered to a patient. The subsequent uptake of the radiotracer is measured over time and used to obtain information about the targeted tissue and gene product. Because of the high-energy (gamma-ray) emissions of the specific isotopes employed and the sensitivity and sophistication of the instruments used to detect them, the two-dimensional distribution of radioactivity may be inferred from outside of the body.
Among the most commonly used positron-emitting nuclides in PET include but are not limited to carbon-11, nitrogen-13, oxygen-15 and fluorine-18. Isotopes that decay by electron capture and/or gamma-emission are used in SPECT, and include but are not limited to iodine-123 and technetium-99m.
A currently used method for labeling amino acids with technetium-99m is the reduction of pertechnetate ion in the presence of a chelating precursor to form the labile technetium-99m-precursor complex, which, in turn, reacts with the metal binding group of a bifunctionally modified chemotactic peptide to form a technetium-99m-chemotactic peptide conjugate.
Antibodies are frequently used for such in vivo imaging diagnostic methods. The preparation and use of antibodies for in vivo diagnosis is well known in the art. For example, antibody-chelators labeled with Indium-111 have been described for use in the radioimmunoscintographic imaging of carcinoembryonic antigen expressing tumors (Sumerdon et al. Nucl. Med. Biol. 1990 17:247-254). Antibody-chelators have been used to detect new metastases and/or tumor recurrence in patients suspected of having recurrent colorectal cancer (Griffin et al. J. Clin. One. 1991 9:631-640). Antibodies with paramagnetic ions as labels for use in magnetic resonance imaging have also been described (Lauffer, R. B. Magnetic Resonance in Medicine 1991 22:339-342). Labeled antibodies which specifically bind a particular molecular biomarker can be injected into patients suspected of having a certain type of cancer, detectable according to that biomarker, for the purpose of diagnosing or staging of the disease status of the patient. The label used will be selected in accordance with the imaging modality to be used as previously described. Localization of the label permits determination of the spread of the cancer. The amount of label within an organ or tissue also allows determination of the presence or absence of cancer in that organ or tissue.
Such techniques may also optionally be performed with labeled oligonucleotides, for example for detection of gene expression through imaging with antisense oligonucleotides (Cherry, Phys Med Biol, vol 29, 2004, pp R13-R48). These methods are already used for in situ hybridization for example with fluorescent molecules or radionuclides as the label. They are currently being developed for use with in vivo imaging. Other methods for detection of gene expression include but are not limited to detection of the activity of a reporter gene.
Another general type of imaging technology is optical imaging, in which fluorescent signals within the subject are detected by an optical device that is external to the subject. These signals may be due to actual fluorescence and/or to bioluminescence. Improvements in the sensitivity of optical detection devices have increased the usefulness of optical imaging for in vivo diagnostic assays (Cherry, Phys Med Biol, vol 29, 2004, pp R13-R48).
The use of in vivo molecular biomarker imaging is increasing, including for clinical trials, for example to more rapidly measure clinical efficacy in trials for new cancer therapies and/or to avoid prolonged treatment with a placebo for those diseases, such as multiple sclerosis, in which such prolonged treatment may be considered to be ethically questionable (Pien et al, DDT, vol 10, February 2005, pp. 259-266).
Another example of diagnostic markers are serum markers which are used for diagnosis of many different diseases. Such serum markers typically encompass secreted proteins and/or peptides; however, some serum markers may be released to the blood upon tissue lysis, such as from myocardial infarction (for example Troponin-I). Serum markers can also be used as risk factors for disease (for example base-line levels of CRP, as a predictor of cardiovascular disease), to monitor disease activity and progression (for example, determination of CRP levels to monitor acute phase inflammatory response) and to predict and monitor drug response (for example, as shedded fragments of the protein Erb-B2).
Immunohistochemistry (IHC) is the study of distribution of an antigen of choice in a sample based on specific antibody-antigen binding, typically on tissue slices. The antibody features a label which can be detected, for example as a stain which is detectable under a microscope. The tissue slices are prepared by being fixed. IHC is therefore particularly suitable for antibody-antigen reactions that are not disturbed or destroyed by the process of fixing the tissue slices.
IHC permits determining the localization of binding, and hence mapping of the presence of the antigen within the tissue and even within different compartments in the cell. Such mapping can provide useful diagnostic information, including:
1) the histological type of the tissue sample
2) the presence of specific cell types within the sample
3) information on the physiological and/or pathological state of cells (e.g. which phase of the cell-cycle they are in)
4) the presence of disease related changes within the sample
5) differentiation between different specific disease subtypes where it is already known the tissue is of disease state (for example, the differentiation between different tumor types when it is already known the sample was taken from cancerous tissue).
IHC information is valuable for more than diagnosis. It can also be used to determine prognosis and therapy treatment (as in the case of HER-2 in breast cancer) and monitor disease.
IHC protein markers could be from any cellular location. Most often these markers are membrane proteins but secreted proteins or intracellular proteins (including intranuclear) can be used as an IHC marker too.
IHC has at least two major disadvantages. It is performed on tissue samples and therefore a tissue sample has to be collected from the patient, which most often requires invasive procedures like biopsy associated with pain, discomfort, hospitalization and risk of infection. In addition, the interpretation of the result is observer dependant and therefore subjective. There is no measured value but rather only an estimation (on a scale of 1-4) of how prevalent the antigen on target is.